During drilling operations, and in particular drilling operations for oil and gas reserves, it is often necessary to break up large-scale downhole material which is present behind the drill bit. One example of large-scale downhole material is parts of a pump-down plug, wiper plug, landing collar, float collar, float shoe etc., which have been used in the process of adding cement around a string of tubular casing.
A casing will be used, for example, to provide support for a particular section of the borehole, i.e. it might be determined that the material surrounding that section of the borehole cannot support the pressure of the drilling fluid or mud which is pumped into the borehole. In such circumstances, it is usual to drill a large-diameter borehole to the depth at which the casing is required, to remove the drill string, to insert a tubular casing which has a diameter slightly smaller than the diameter of the borehole, and to pump a cementitious material into the annular gap between the casing and the borehole wall. The drilling operation continues with a smaller-diameter drill passing through the casing.
It is not uncommon for a borehole to be formed by a series of reducing-diameter drilled sections, with each section lined by a string of tubular casing or tubular liner.
To prevent the casing filling with dirt and debris during its insertion into the borehole, its bottom end may be fitted with a shoe and float valve so that it becomes necessary for the drill to break up the shoe and float valve before drilling of the next borehole section can proceed. In addition, the casing will typically be fitted with a landing collar to receive at least two plugs, the first of which precedes the cement as this is pumped into the casing, and the second of which follows the cement, the plugs separating the cement from the drilling fluid within the casing. Typically, the first plug will contain a valve which opens when the plug engages the shoe at the bottom of the casing, allowing cement through the plug and into the volume between the casing and the borehole.
The distance between the plugs, and the volume of concrete which can be contained within the casing between the plugs, will be calculated in advance based upon the volume of cement required to fill the annular gap around the casing.
It is also necessary to drill out the plugs and landing collar before the drilling operation can continue.
The borehole will typically be drilled to a depth slightly greater than that required for the casing, so that the casing does not reach the bottom of the borehole. During the cementing stage, it is common for the bottom of the borehole (i.e. that region below the bottom of the casing, and which is often referred to as the “rat hole”) to be partially or totally filled with cement and other debris. The material which is located in the rat hole must also be drilled out before the drilling operation can continue.
The plugs, landing collar, float valves and shoe are made of a material, and the rat hole may contain a material, different to that through which the borehole is being drilled. The drill bit is suited to drilling a particular material such as rock, and whilst it is possible to fit a specialised drill bit for the sole purpose of breaking up the plugs, landing collar, float valves, shoe and material within the rat hole, this is usually not economic as it can for example take up to 1-2 days to pass the specialised drill bit down to a typical casing shoe depth, drill out the plugs, landing collar, float valves, shoe and rat hole, return the specialised drill bit to the surface, and then introduce the regular drill bit. With the cost of 1 day's use of a drilling rig being typically in the hundreds of thousands of US Dollars, it is preferable to avoid unnecessary trips up and down the borehole. Accordingly, a specialised drill bit is not often used, and instead a drill bit and drilling assembly which are suited to the surrounding rock or earth are used to penetrate the plugs, landing collar, float valves, shoe and contents of the rat hole before they can continue drilling into the rock or earth.
It is known to make the plugs from a friable material, i.e. a material which is susceptible to drilling by the regular drill bit, and which is designed to break up into small-scale pieces when drilled. However, such plugs are not in universal use, and many applications utilise plugs made from a combination of aluminium and rubber. It is a widely recognised disadvantage with plugs of this type that the drill bit will not break up these materials very efficiently, with the ductility of the aluminium, and the resilience of the rubber, allowing these materials to remain as large-scale materials in the form of ribbons, particles or chunks as they pass the drill bit.
The existence of large ribbons, particles or chunks of aluminium, rubber and the like can cause significant damage to other components of the downhole assembly. For example, the downhole assembly might include a steering component such as that described in our published patent application EP-A-1024245, and whilst that steering component (and other downhole componentry) is adapted to the downhole conditions including drilling fluids and entrained drill cuttings, the drill cuttings are usually small-sized particles, and the component may not be able to function in the presence of large-scale materials.
Specifically, the ribbons, large particles and/or chunks of aluminium, rubber and the like can foul parts of the steering component (e.g. become wedged or jammed against the component) and cause mechanical damage, and/or they can cause a pack-off, i.e. block the passage of drilling fluid around the steering component. The blockage of drilling fluid, even for a very short period of time, can result in a very large pressure drop (perhaps above 5,000 p.s.i. (approx 33×106 Pa)) across the steering component, which can lead to failure of the seals and the subsequent ingress of drilling fluid, for example.